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When biomass is gasified using air as the oxidizer, the gas produced has a low energy value. That's primarily because air is only 20% efficient as an oxidizer, with the result that about half of wood gas produced using air is inert nitrogen. That's alright when the wood gas is being used to fuel an internal combustion engine as it's being produced.
by Vince Harlow
The Complete and Unauthorized Sourcebook on the Handling and Use of Oxygen, Nitrox and Trimix, for Dive, Aviation and Emergency Use.
But all that inert nitrogen is a real problem when the goal is to store a gaseous fuel or to convert it into a liquid form. As a result, a key step in the conversion of biomass into a fuel that can be economically stored or transported involves the use of oxygen to unleash the energy that's bound up in biomass.
For almost a century, pure oxygen is produced by the fractional distillation of liquid air, an energy-intensive process which entails a wide range of challenges. Back in the 1970's, when Union Carbide developed the PureOx process for converting municipal solid waste (MSW) into a combustible gas, the cost of the liquid air plant was a major part of the anticipated expense of setting up the facility.
Because of that cost, the size of the MSW plant needed to be substantial as well. In order to feed such a facility, large volumes of waste would need to be hauled to the plant from considerable distances, and historically it's been the cost of that transportation that has been the downfall of large biomass-to-energy systems.
In the case of the Seattle project, the scale of the project drew lots of opposition from people who didn't want lots of garbage trucks hauling waste into their community. One of the advantages of the B2M approach is that, because such a facility is scaled to deal with only the biomass produced by the village, the cost of loading, hauling and unloading large amounts of biomess is saved.
In the 1990's the development of pressure swing adsorption (PSA) units made it possible to economically generate small quantities of 95% pure oxygen (O95) from lightly compressed (1 bar/15 psi) air. [note: the remaining 5% is argon, an inert gas.]
This break-through enabled the in-home generation of oxygen for patients who depend on supplemental oxygen, a large market that provided a base for the commercial development of the PSA technology. That work then led to the development of larger PSAs that could be used to raise the dissolved oxygen content of water in commercial fish farms, thereby allowing more fish to be grown in a given tank.
As a result of these developments in home-based medical care and in commercial aquaponic facilities, PSA technology has gone a long way towards making possible the village scale conversion of locally generated biomass into liquid fuels.
The use of highly-compressed pure oxygen is routine in workshops, garages and on farms, but that's not to say that it's to be taken lightly. There are well established safety procedures, but they need to be understood and complied with any time one is using any concentrated form of energy.
Please note that it is not the intention of this blog to serve as a manual on the safe use of concentrated energy forms, but rather to serve as a clearing house that will point the would-be gasification student in the direction they need to go. Harlow's manual is a good example of a resource containing that sort of hard-to-find information.
One of the problems that makes it difficult to talk about the proper use of oxygen is that oxygen is the standard by which all other oxidizers are measured. To be precise, oxygen doesn't burn, but that most everything else will burn in its presence. If you've ever seen a cutting torch in operation, what's happening is that pure oxygen is coming into contact with hot steel and setting it on fire. It's the heat given off by the burning steel that actually does the cutting.
Those who want to explore the challenges and potential benefits that can come from using O95 need to invest considerable effort before before hand learning how to use it properly, but such information is difficult to come by. Vance Harlow's manual is one of the best sources of information on handling compressed oxygen available. His application (specialty gas mixes for use in deep water diving) is different from the work involved in B2M, but many of the concepts and precautions Harlow describes are directly relevant.
Additionally, Harlow's manual is a great introduction to the various fittings and materials that have been developed to ensure the safe and reliable use of pure oxygen. While it may be tempting to take short cuts, that's a fool's game; far better to take whatever time is needed to track down the proper fittings and materials. Marlow's manual makes this much easier.
To visit Harlow's website, Click Here